linux-kernel - Re: [PATCH] swiotlb: Fix unexpected swiotlb_alloc_coherent() failures

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Date:   Tue, 10 Apr 2018 18:50:04 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Takashi Iwai <tiwai@...e.de>, Christoph Hellwig <hch@....de>
Cc:     linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        Christian König <christian.koenig@....com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH] swiotlb: Fix unexpected swiotlb_alloc_coherent() failures

On 10/04/18 18:07, Takashi Iwai wrote:
> On Tue, 10 Apr 2018 19:06:15 +0200,
> Christoph Hellwig wrote:
>>
>> On Tue, Apr 10, 2018 at 07:05:13PM +0200, Takashi Iwai wrote:
>>> The code refactoring by commit 0176adb00406 ("swiotlb: refactor
>>> coherent buffer allocation") made swiotlb_alloc_buffer() almost always
>>> failing due to a thinko: namely, the function evaluates the
>>> dma_coherent_ok() call incorrectly and dealing as if it's invalid.
>>> This ends up with weird errors like iwlwifi probe failure or amdgpu
>>> screen flickering.
>>>
>>> This patch corrects the logic error.
>>
>> This looks ok, although even hitting this code is a bad sign.  Do you
>> know why the previous dma_direct_alloc didn't succeed?
> 
> That I have no idea, sorry.  I just figured out the regression just
> from the dmesg output of 4.16 kernel in bugzilla reports.
> Could you take a look at the bugzilla reports there?

In the first one, the machine appears to have enough RAM that most of it 
is beyond the device's 36-bit DMA mask, thus it's fairly likely for the 
initial direct alloc to come back with something unsuitable.

Robin.

> 
> 
> thanks,
> 
> Takashi
> 
>>> Bugzilla: https://bugzilla.suse.com/show_bug.cgi?id=1088658
>>> Bugzilla: https://bugzilla.suse.com/show_bug.cgi?id=1088902
>>> Fixes: 0176adb00406 ("swiotlb: refactor coherent buffer allocation")
>>> Cc: <stable@...r.kernel.org> # v4.16+
>>> Signed-off-by: Takashi Iwai <tiwai@...e.de>
>>> ---
>>>   lib/swiotlb.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/lib/swiotlb.c b/lib/swiotlb.c
>>> index 47aeb04c1997..de7cc540450f 100644
>>> --- a/lib/swiotlb.c
>>> +++ b/lib/swiotlb.c
>>> @@ -719,7 +719,7 @@ swiotlb_alloc_buffer(struct device *dev, size_t size, dma_addr_t *dma_handle,
>>>   		goto out_warn;
>>>   
>>>   	*dma_handle = __phys_to_dma(dev, phys_addr);
>>> -	if (dma_coherent_ok(dev, *dma_handle, size))
>>> +	if (!dma_coherent_ok(dev, *dma_handle, size))
>>>   		goto out_unmap;
>>>   
>>>   	memset(phys_to_virt(phys_addr), 0, size);
>>> -- 
>>> 2.16.3
>> ---end quoted text---
>>
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> iommu@...ts.linux-foundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/iommu
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